OK more issues - FUEL??? 1975 - 70 hp

OK this Sunday i took the kids tubing and had a great time. We stopped at the beach anchored for an hour and then .... dead. Everything was working fine in the morning. Once stopped I did put a little bit of SEAFOAM in the gas tank as the motor has been sitting for a good 4 years before this summer.
I took the plugs out (new this year) and cleaned them (2 of the 3 were not dirty and the one that was a little dark still I would consider OK.) I changed out the fuel to nice new clean mid grad gas with 50:1 oil mixture. So now I can get the engine running in neutral at full throttle but it is not reaching full revs. I checked the fuel filter and it looks clean. What else do I need to check???? What could the SEAFOAM have done to the engine to make it run like this? The gas can is under the center console about 10' of gas line.
PLEASE HELP I AM GETTING FRUSTRATED.....

Sitting that length of time...... the carburetors are fouled. Remove, clean, and rebuild them.

Compression test maybe? Last time t-stat was checked changed? Fuel pump maybe clogged a little.

All very easy to do and except for the price of the t-stat and gauge free to do. Maybe you already have a gauge.

Good luck i have that exact motor.

I meant to mention....... Running a engine at full throttle or just a very fast rpm when in neutral is a excellent way to encounter a runaway engine. It will simply increase its rpms even with the throttle closed. By the time you think of a way to shut it down, connecting rods will be flying out the side of the crankcase.

Zamboni65

did you get your issue fixed ?